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AN-53 Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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AN-53 Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 20 page  AN53 APPLICATION NOTE 10 When using these formulae, special care must be taken regarding the MOSFETs’ transition times: the rise and fall refer to the MOSFETs’ drain-source voltage, NOT the gate- source. Using the datasheet values (rather than measured val- ues) can also result in serious overestimation of the losses, since the transition is being driven by an inductive source, not a resistor. Selecting the Inductor The inductor is one of the most critical components to be selected for a DC-DC converter application. The critical param- eters of the inductor are its inductance (L), maximum DC cur- rent (IO ), and DC coil resistance (Rl ). The inductor’s inductance helps determine two key parame- ters of a converter, its ripple current and its transient response. On the one hand, making the inductance large reduces the ripple current, and thus the output ripple voltage. On the other hand, a large inductance provides a slow response to load transients. For Pentium II supplies, the tran- sient response is paramount, and thus the inductance is typi- cally chosen to be in the 1-5 mH range. Most inductors’ inductance also depends on current, that is, increasing the current through the inductor decreases the inductance. It is thus vital to specify the DC current when procuring an inductor. The one type of inductor which does not change inductance with current is the rod-core inductor, but this type may have significant EMI (noise) problems. For further information, refer to Applications Bulletin AB-12. The resistance of the winding of the inductor is also impor- tant, as it is directly responsible for much of the losses in the inductor. Minimizing the resistance will help improve the converter’s efficiency. Implementing Over-current Protection Intel currently requires all power supply manufacturers to provide continuous protection against short circuit condi- tions that may damage the CPU. To address this requirement, Fairchild Semiconductor has implemented a current sense methodology to limit the power delivered to the load in the event of over-current. The voltage drop created by the output current across a sense resistor is presented to one terminal of an internal comparator with hysteresis. The other comparator terminal has the threshold voltage, nominally of 120mV. Table 7 states the limits for the comparator threshold of the Switching Regulator. Table 7. RC5051 Over-current Comparator Threshold Voltage When designing the external current sense circuitry, pay careful attention to the output limitations during normal operation and during a fault condition. If the over-current protection threshold current is set too low, the DC-DC con- verter may not be able to continuously deliver the maximum CPU load current. If the threshold level is too high, the out- put driver may not be disabled at a safe limit and the result- ing power dissipation within the MOSFETs may rise to destructive levels. The following is the design equation used to set the over-current threshold limit: Where I pk is defined as in Figure 7, and Iload, max = maximum output load current. Figure 7 illustrates the inductor current waveform for the RC5051 DC-DC converter at maximum load. Figure 7. Typical DC-DC Converter Inductor Current Waveform The calculation of the ripple current is as follows: where: VIN = input voltage to converter, TS = the switching period of the converter = 1/fS, and fS = switching frequency. As an example, for an input voltage of 5V, output voltage of 2.8V @ 14A, L equal to 1.3 mH and a switching frequency of 285KHz (using CEXT = 100pF), the peak inductor current can be calculated as : Therefore, the over-current detection threshold must be at least 16A. The next step is to determine the value of the sense resistor. Including sense resistor tolerance, the sense resistor value can be determined as Where TF = Tolerance Factor for the sense resistor. Table 8 describes tolerance, size, power capability, temperature coef- ficient and cost of various type of sense resistors. Short Circuit Comparator Vthreshold (mV) Typical 120 Minimum 100 Maximum 140 I PK I LOAD MAX , = I RIPPLE 2 ------------------- + t I Ts T ON IPK IMIN T OFF I LOAD, MAX 65-AP53-04 I RIPPLE 2 ------------------- V IN V OUT – 2L ------------------------------T S V OUT V IN -------------- = I PK 14A 5V 2.8V – 2 1.3 mH ´ ------------------------- 1 285kHz -------------------- ´ 2.8V 5V ------------ ´ + 15.7A == R SENSE V th,min I SC 1TF + () ------------------------------ = |
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Similar Description - AN-53 |
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